Outbalanced: The cross-cortical effects of prefrontal neuromodulation in posterior parietal cortex

Cognitive phenomena such as the Spatial–Numerical Association of Response Codes (SNARC) effect can arise in the fronto-parietal cortical network. Prior neuromodulation studies with cathodal transcranial direct current stimulation (tDCS) over the left prefrontal cortex (PFC) reduced the SNARC effect. Prior neuroimaging studies with functional near-infrared spectroscopy (fNIRS), however, showed signatures of the SNARC effect in the posterior parietal cortex (PPC). In this study, we investigated the distant neural effect of prefrontal neuromodulation on hemodynamic activity in the parietal cortex by combining cathodal tDCS with fNIRS. The SNARC effect and the numerical distance effect (NDE) were assessed in an event-related cross-over design (N = 45), when cathodal tDCS of 1 mA at the left PFC was applied simultaneously during the measurement of fNIRS covering the bilateral PPC. At the behavioral level, prefrontal tDCS did not significantly reduce the SNARC effect, indicating that the replication failed here. Crucially, at the neuronal level, prefrontal tDCS reduced left parietal activation associated with the SNARC effect but not with the NDE. This neuronal effect of tDCS in a remote site was shown in preregistered primary region-of-interest analyses and in secondary all-channel analyses. The results showed how the combination of neuromodulation and neuroimaging shed light on the fronto-parietal network responsible for numerical cognition, and how fNIRS can assess the distant neural effects of cathodal tDCS.